The tire retreading industry has become a highly competitive field for producing vehicle tires using old carcasses with new treads vulcanized thereto. The success in this field dictates that with the current labor intensive retreading techniques, the rejects or defective recapped tires be maintained at an absolute minimum. This is important both from a standpoint of profit margin, as well as liability.
One current technique utilized throughout the major portion of the industry is to apply a new tread to an old carcass which has had the old tread removed, such as by buffing or other abrasive means. After the removal of the old tread, a layer of uncured raw rubber is applied over the crown of the carcass. The new retread is then stretched over the rubber layer and adjusted on the crown of the tire to assure proper alignment with the carcass. A sealing envelope is placed around the recapped portion of the carcass and held against the tire sidewalls to seal the new retread from the moisture of the vulcanizing environment. Air is then evacuated from between the tire and the sealed envelope.
The tire carcass and new tread are then vulcanized to each other in a high pressure and high temperature atmosphere. In many cases, steam is used to accelerate the vulcanizing process. In order to assure that air pockets or moisture do not form between the carcass and the new tread, it is imperative that the envelope be sealed against the carcass sidewalls during the entire vulcanizing process.
Many techniques have been developed in an attempt to assure that a reliable seal is maintained between the envelope and tire carcass. For example, disclosed in U.S. Pat. No. 4,274,897 is a pair of wire grids or grills disposed on each side of the tire carcass for pressing the envelope edges against the carcass sidewalls. Coil springs with hooked ends are used to pull the grills together, thereby compressing the grills and the envelope against tire sidewall. Disclosed also is the auxiliary use of three or four small spacers for spacing the tire beads apart. With the mechanism disclosed in the noted patent, the sealing ability of the grills are dependent in a large part on the stiffness of the tire carcass and its ability to resist compressive forces.
U.S. Pat. No. 4,153,497 discloses a tire retreading apparatus with two annular rings which press the envelope against tire sidewalls. The rings are maintained a predetermined distance apart. Four hooked bolts are engageable both with a spider assembly and with the tire beads at four corresponding points. The hooked bolts pull the tire beads outwardly and thus force the sidewalls against the annular rings. The envelope is thus sealed against the tire sidewalls. With this apparatus, the application of a compressive force to the sidewalls is made using only four points along the tire beads. This approach presents the possibility of permanently bending the tire bead if the hooked bolts are tightened excessively. Again, the assurance of a good seal depends on the stiffness of the tire beads as well as the stiffness of the tire sidewalls.
With the increased use of radial tires having resilient sidewalls, it may be seen that a need has arisen for an improved tire retreading apparatus which provides a better compressive force that is uniform along the entire circumference of the sidewalls, and which is more independent of the stiffness of the tire sidewalls. There is an associated need for retreading apparatus having a hub-type support for the tire carcass during the retreading process, without the inconvenience of positioning a tube inside the carcass and installing the tire carcass on a rimmed hub structure.